The catalysts for the hydrotreating of hydrocarbon feedstocks to which the present invention relates are used, under appropriate conditions, for converting, in the presence of hydrogen, organosulphur compounds to hydrogen sulphide, which operation is known as hydrodesulphurization (HDS), and for converting organonitrogen compounds to ammonia in an operation known as hydrodenitrogenation (HDN).
These catalysts are generally based on metals from Groups VI B and VIII of the Periodic Table of the Elements, such as molybdenum, tungsten, nickel and cobalt. The most commonly used hydrotreating catalysts are formulated from cobalt-molybdenum (Co—Mo), nickel-molybdenum (Ni—Mo) and nickel-tungsten (Ni—W) systems, deposited on porous inorganic supports, such as aluminas, silicas or silicas/aluminas. These catalysts, manufactured industrially at very large tonnages, are supplied to the user in their oxide forms (for example, catalysts formed of cobalt oxide-molybdenum oxide on alumina, symbolized by the abbreviation: Co—Mo/alumina).
However, these catalysts are active in hydrotreating operations only in the form of metal sulphides. This is why, before being used, they have to be sulphided.
As regards the activation of hydrotreating catalysts, the sulphidation of these catalysts is an important stage in maximizing their performances in EMS and in HDN. As is indicated by the authors of Hydrotreating Catalysis (Catalysis, vol. 11, 1996, p. 25, edited by J. R. Anderson and M. Boudart), practical experience has shown that the sulphidation procedure can have a significant influence on the activity and the stability of the catalyst and great efforts have been devoted to improving the sulphidation procedures.
The most direct method for the sulphidation of a catalyst consists in treating the latter with hydrogen sulphide mixed with hydrogen. However, this method, which has formed the subject-matter of numerous patents (U.S. Pat. No. 3,016,347, U.S. Pat. No. 3,140,994, GB 1 309 457, U.S. Pat. No. 3,732,155, U.S. Pat. No. 4,098,682, U.S. Pat. No. 4,132,632, U.S. Pat. No. 4,172,027, U.S. Pat. No. 4,176,087, U.S. Pat. No. 4,334,982, FR 2 476 118), exhibits major disadvantages (acute toxicity, difficulty in supplying H2S) which do not allow it to be employed on all industrial sites.
Industrial procedures for the sulphidation of catalysts are generally carried out under hydrogen pressure with liquid feedstocks already comprising sulphur compounds as sulphiding agents. The main method used in the past by refiners consisted in sulphiding the catalysts with sulphur-comprising petroleum feedstocks but this technique exhibited significant disadvantages because of the difficulty of converting the sulphur compounds to hydrogen sulphide. In order to prevent the catalysts being reduced by the hydrogen, the sulphidations, begun at low temperature, had to be taken slowly to high temperature in order to obtain complete sulphidation of the catalysts.
Sulphur-comprising additives have been provided for improving the sulphidation of the catalysts. The method consists in incorporating a sulphur compound (spiking agent) in a feedstock, such as a naphtha, or in a specific fraction, such as a VGO (vacuum gas oil) or an LGO (light gas oil).
The use of sulphur compounds in the non-oxidized form was claimed for the first time in U.S. Pat. No. 3,140,994, in particular: carbon disulphide, thiophene, mercaptans and organic sulphides, in particular dialkyl disulphides or diaryl disulphides, dimethyl disulphide (DMDS) being exemplified.
EP 64 429 describes an effective method for sulphidation by means of a sulphidation feedstock composed of a mixture of at least one sulphur compound and a hydrocarbon feedstock and of a specific temperature profile; mention is made, among sulphur compounds, of carbon disulphide, mercaptans, thiophene compounds, (di)sulphides and hydrogen sulphide, dimethyl disulphide (DMDS) being particularly preferred for the sulphidation of the catalysts and with dimethyl disulphide is described in the patent.
H. Hallie (Oil and Gas Journal, Dec. 20, 1982, pp 69-74) has reviewed these procedures for sulphidation under hydrogen which are carried out directly in hydrotreating reactors. These various techniques for the sulphidation of catalysts, known as “in-situ” techniques, have been compared and studies have shown that sulphidation with a liquid feedstock to which has been added a sulphiding agent (spiked feedstock) which has the property of decomposing at low temperature is the best sulphidation technique. The technique without an additional sulphiding agent (nonspiked feedstock) gives a less active sulphided catalyst. The sulphiding agent which it is preferred to add to the feedstock is dimethyl disulphide.
It is known to a person skilled in the art that dialkyl disulphides can be used as sulphiding agents; however, only dimethyl disulphide has been explicitly mentioned as sulphiding agent, dimethyl disulphide moreover being the reference sulphiding agent in the industry to date.
Organic polysulphides have also been recommended as sulphiding agents for the sulphidation of catalysts. U.S. Pat. No. 4,725,569 describes the use of an organic polysulphide of RxR′ type (R and R′ being C1-C20 alkyl groups which can be identical or different, with x between 2 and 8, DMDS being excluded) which consists in impregnating the catalyst at ambient temperature with a solution comprising the polysulphide, in subsequently removing the inert solvent and, finally, in carrying out the sulphidation, under hydrogen, of the catalyst charged to the hydrotreating reactor.
EP 298 111 describes a process for the sulphidation of a catalyst by simultaneously passing hydrogen and a hydrocarbon feedstock comprising a sulphiding agent of formula RSnR′ (R and R′ being C1-C4 alkyl radicals which can be identical or different, with n between 3 and 10).
WO 01/96499 describes the use as sulphiding agent of mixtures of disulphides resulting from an LPG (liquefied petroleum gas) desulphurization unit of an oil refining unit, the caustic and sodium compounds having been removed from these mixtures. These mixtures of disulphides generally consist, to greater than 98%, of dimethyl disulphide, diethyl disulphide and ethyl methyl disulphide.
EP 0 976 726 describes a composition based on DMDS with a masked odour comprising up to 1% by weight of an odour-masking agent chosen from vanillin, ethyl vanillin and some esters. This masking is effective only if the content of impurities in the DMDS is limited, typically less than 500 ppm of methyl mercaptan, less than 1% of dimethyl sulphide.
It is known to a person skilled in the art that alkyl polysulphides RSxR′ (x being the mean sulphur value and x≧3) decompose at a lower temperature than alkyl disulphides, such as DMDS, which exhibits the advantage of making possible faster sulphidation of the catalysts, an advantage which the industry turns to good account. However, the major disadvantage of polysulphides, also well known in the industry, is the formation of solid sulphur and/or of a solid deposit which is generated during the heat treatment for activation of the catalyst; the solid sulphur and/or the solid deposit can be deposited in the various components of the refinery and thus create blockages which are very harmful to the operation of the industrial unit. Although polysulphides decompose at a lower temperature than alkyl disulphides, the formation of a solid deposit and/or of solid sulphur related to their use represents a problem for refiners who their prefers dimethyl disulphide, which remains the reference sulphiding agent in the industry.
Novel techniques for the sulphiding of catalysts comprising two stages have recently been proposed. In a first stage, referred to as “ex situ”, the catalyst is preactivated in the absence of hydrogen outside the refinery after having been impregnated with a sulphiding agent.
EP 130 850 describes a process for the ex situ presulphidation of a catalyst which consists in treating the said catalyst using at least one sulphiding agent of typical formula RSnR′ (R and R′ being C1-C150 organic radicals (alkyl, naphthenic, aryl, alkylaryl, arylalkyl) which may be identical or different, with n between 3 and 20) used in solution in a solvent; the catalyst in the oxide form is impregnated with a solution of organic polysulphides (for example TPS 37 or TNPS, sold by Arkema), preferably in a hydrocarbon of white spirit type. This preliminary stage of incorporation in the catalyst of a sulphur compound of specific nature is completed by heat treatment of the catalyst in the absence of hydrogen at temperatures not exceeding 150° C. This operation has the effect of removing the organic solvent and of ensuring the attachment of the sulphur to the catalyst via organic polysulphides. At this presulphidation stage, the catalyst is stable in air and can be handled without specific precautions. It is supplied in this state to the user who, after charging to the hydrotreating reactor, can complete the sulphidation of the catalyst under hydrogen for the complete conversion of the metals to metal sulphides in the hydrotreating reactor in the presence of hydrogen. The “ex situ” techniques currently developed on the industrial scale use, as sulphur-comprising products, organic polysulphides or sulphur.
Other organic polysulphide compounds, with different structures, have also been proposed for the “ex situ” presulphidation of catalysts. The products recommended in FR 2 627 104 and EP 329 499 have the general formula R′—(Sy—R—Sx—R—Sy)—R′ and are obtained from olefins and sulphur chloride by a series of successive stages which involve a reaction with an organic monohalide followed by a reaction with an alkaline polysulphide. In EP 338 897, the claimed products are synthesized from olefins and sulphur chloride with an additional reaction with an alkaline mercaptide or an alkaline polysulphide mercaptate.